Today, response to global environmental issues such as CO2 restrictions is an urgent matter in the automotive industry. On the other hand, from the viewpoint of ensuring safety of passengers, collision safety standards of automobiles have been strengthened, and there has been progressing a structural design for sufficiently ensuring safety in an in-vehicle space. To achieve such demands together, it is effective that a high-strength steel sheet (high-tensile steel sheet) having a tensile strength of at least 980 MPa is used for structural members of the automobile, and is reduced in thickness to lighten a vehicle body. However, higher strength of a steel sheet in general leads to degradation of workability. Hence, when a high-tensile steel sheet is applied to automobile members, improving workability of the steel sheet is an inevitable issue.
Known steel sheets having both strength and workability include TRIP (Transformation Induced Plasticity) steel sheets. A TBF steel sheet including bainitic ferrite as a parent phase and retained austenite is known as one of the TRIP steel sheets (PTL 1 to PTL 4). The TBF steel sheet has high strength due to the hard bainitic ferrite, and has excellent elongation (EL) and stretch-flangeability (λ) due to the fine retained-austenite located in a grain boundary of the bainitic ferrite, and therefore has high strength and excellent workability together.
PTL 5 discloses a method of manufacturing a high-strength steel sheet having a tensile strength of at least 980 MPa and having good elongation and good stretch-flangeability. In this manufacturing method, a steel sheet containing at least 0.10 mass % of C is heated into an austenite single-phase region or a duplex (austenite and ferrite) region. Subsequently, the steel sheet is cooled to a target cooling end temperature that is set, with a martensitic transformation start temperature Ms as an index, in a temperature region of lower than Ms and at least Ms−150° C. Thus, part of untransformed austenite is transformed into martensite, and then the steel sheet is heated to temper the martensite.